1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium with a thin ferromagnetic film being vacuum-deposited on a moving substrate as a magnetic recording layer. More particularly, the present invention relates to a process for producing a magnetic recording medium having superior weatherproofness and improved electromagnetic conversion characteristics.
2. Background of the Invention
Commonly employed magnetic recording media are of the coated type and are manufactured as follows. Magnetic particles are used that are in the form of an oxide powder made of, for example, .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, Co-doped Berthollide compound, or CrO.sub.2, or in the from of an alloy powder based on, for example, Fe, Co or Ni. These magnetic particles are dispersed in organic binders such as a vinyl chloride/vinylacetate copolymer, a styrene/butadiene copolymer, an epoxy resin or a polyurethane resin. The resulting magnetic paint is applied to a nonmagnetic substrate and dried.
With the growing need for higher-density magnetic recording thin films of ferromagnetic metals formed by such methods as evaporation, sputtering and ion plating have attracted researchers' attention because of their utility in binder-free magnetic recording media of the "thin metal film type" and active efforts are being made to commercialize them.
The conventional magnetic recording media of the coated type are not adapted to recording for large output and at high density because the magnetic materials they chiefly use are formed of metal oxides having small saturation magnetization. Additionally the magnetic material accounts for only 30-50% of the volume of the magnetic layer. The coated-type media have the additional disadvantage that their manufacturing process is complicated and requires costly equipment for solvent recovery or pollution control. On the other hand, magnetic recording media of the thin metal film type have the advantage that ferromagnetic films having a larger saturation magnetization than oxide-based magnetic materials can be formed as extremely thin films without using any nonmagnetic materials such as organic binders. The practice of high-density recording has also increased the use of a playback head having a gap length of not more than 1.0 micrometer and this in turn has created the tendency that the depth of recording in the magnetic recording layer is reduced. Under these circumstances, magnetic recording media of the thin metal film type which permits the entire thickness of the magnetic film to be utilized in the recording of magnetic information are best suited for use in large-output and high-density recording. While thin metal-film type media can be manufactured by various methods, vacuum evaporation has the advantages that it achieves rapid film formation, requires a simple manufacturing scheme, and is a dry process which involves no need for effluent treatment. It has been proposed that a beam of evaporated magnetic metal be deposited at an angle on the surface of a nonmagnetic support and this method is highly advantageous for practical purposes since it requires comparatively simple manufacturing process and equipment and enables the production of films having good magnetic characteristics.
However, magnetic recording madia that employ thin films of ferromagnetic metals formed by evaporation have one major problem associated with weather-proofness. No magnetic recording medium that is being stored should experience any attenuation or loss of the recorded signal on account of corrosion or otherwise initiated time-dependent deterioration. In addition, 8-mm video tapes are in most cases used outdoor and the magnetic recording media of which they are formed must have satisfactory weather-proofness under the various environmental conditions in which they are used. Although the magnetic recording media that employ thin films of ferromagnetic metals formed by evaporation exhibit superior electromagnetic conversion characteristics over the conventional coated-type media, the recent improvement in the characteristics of the latter type has been remarkable and this calls for further improvements in the electromagnetic conversion characteristics of the evaporated-type media.
While various proposals have been made in order to improve the weatherproofness of magnetic recording media that employ thin deposited-vapor films of ferromagnetic metals as recording layers, one method is described in Japanese Patent Application (OPI) Nos. 198543/1982 and 17544/1983 (the term OPI as used hereinafter means an unexamined published Japanese patent application). According to this method, a thin film of ferromagnetic metal is vapor-deposited on a nonmagnetic substrate travelling on the periphery of the lateral side of a rotating cylinder. Thereafter, the surface of the thin film while being guided by the same cylinder is exposed to a glow discharging atmosphere made of an oxidizing gas. This method is effective to some extent in reducing the decrease in remanence that will occur during storage in a hot and humid atmosphere but the improvement achieved in this respect is not completely satisfactory. In addition, this method is ineffective for the purpose of providing improved electromagnetic conversion characteristics.
The supplying of oxygen simultaneously with vacuumdeposition of ferromagnetic materials at an angle has been proposed in patent references such as Japanese Patent Application (OPI) Nos. 32234/1983, 37843/1983, 41439/1983, 41442/1983, 41443/1983, 53027/1983, 45625/1983 and 157717/1985. However, none of the methods proposed in these references have achieved satisfactory improvements in weatherproofness or electromagnetic conversion characteristics.